1. Technical Field
The present disclosure generally relates to the field of illumination devices and, more particularly, to power monitoring of illumination devices using switch-mode power converters.
2. Description of the Related Art
Power measurement has traditionally relied upon measuring the current and voltage supplied to a load. Typical voltage measurement involves the use of an analog to digital converter. Typical current measurement involves the measurement of the voltage across a current sense resistor, or current sensing transformer, using an analog to digital converter. Because the alternating current used in many municipal, commercial, and industrial lighting systems is not supplied as a perfect sinusoid, the calculation of power is not as straightforward as a simple multiplication of the voltage and current. In practice, a digital signal processor is often employed due to the need for high speed data acquisition, noise filtering, root-mean-square conversion, and power computation. It is difficult to measure the current accurately because of the phase reversal of the AC waveform, and the need for the measurement to be “floating” relative to the power measurement system's direct current (DC) power supply. Application specific integrated circuits (ASICs) are available for the analog front end (AFE) of traditional power meters.
Automated power consumption measurement of devices powered by switch-mode type electronic converters has become an area of interest to electric utilities and others. For example, street lights using switch-mode power converters are now connected by communication networks such as 802.11 (WiFi) networks, power line carrier networks, or others. Such networks may be used by utilities and other operators to monitor the energy consumption, state of operation, and various other parameters of the various illumination devices within the network.
Switch-mode power converters can convert either a DC or rectified AC input voltage to a DC output voltage. The DC or relatively low frequency AC (e.g., 50-60 Hz) input voltage is converted to a high frequency (e.g., 40-60 kHz) pulse output. The conversion to a high frequency pulse affords two benefits. First, the physical size the transformer and capacitance in the power converter is reduced. Second, the impact of variations in the input voltage and variations in a load driven by the output voltage are minimized.
Typically, a switch-mode power converter includes an inductor and a switching device (usually a semiconductor switching device such as a multi-stage metal oxide semiconductor field effect transistor or MOSFET) coupled in series and connected to the DC or rectified AC input voltage source. A drive signal, such as a pulse width modulated signal drives the switching device to sequentially cause the storage and discharge energy in the inductor. Where the inductor is provided by a transformer primary, the transformer secondary can be coupled to the load. A capacitor and an optional inductor may be coupled across the output of the switch mode power converter to further level, filter and stabilize the DC output voltage delivered to the load.
Through the use of a semiconductor switching device, a very high efficiency can be achieved within the switch-mode power supply since small switching losses occur with high speed, low drive current semiconductor switches. By adjusting the conductive period of the switching device relative to the total operating cycle, the DC output voltage can be maintained at virtually any constant value. Feedback control is used to alter or adjust the conductive period of the switching device such that the power delivered at the output of the switch-mode power converter approximates a constant voltage or constant current even with varying power demand of the connected load or changes in line voltage. By supplying only the power needed by the load, the efficiency of the converter is further increased. With the expanding use of solid state light sources in municipal, commercial and industrial settings, the ability to accurately, efficiently, and economically measure the power consumption of such devices is increasing.
New, more efficient, approaches to improving power measurement in devices using switch-mode electronic converters are therefore desirable.